To an increasing extent, electronic circuits in the automotive field must be able to fulfill a full or restricted scope of functions (sending messages to telephone modules for emergency calls, deployment of airbags, etc.) for a certain period of time (reserve power time) after the vehicle electric supply voltage has been shut down or the battery has been disconnected (e.g., in a collision).
In control units today, the power required for this is typically stored temporarily in a capacitor. According to the equation W=½C U2, this power is proportional to capacitance C of the capacitor and the square of voltage U. To minimize capacitance C of the capacitor and be able to store a large amount of power, the capacitor is usually charged to a voltage which is higher than the vehicle electric system supply voltage, via a step-up regulator, which is generally designed as a switching regulator.
In the event of loss of power supply voltage, power is taken from the reserve energy capacitor via one or more step-down regulators which generate the required internal normal d.c. voltage(s).
This is explained in greater detail below with reference to FIG. 1 of the drawing.
FIG. 1 shows a circuit arrangement known from the related art in a highly schematized style, with vehicle electric system supply voltage VBAT being supplied to the voltage input at the left in the figure via a non-reversible diode 1 and the voltage output at the right in the figure delivering an internal normal d.c. voltage VCC which is usually lower than vehicle electric system supply voltage VBAT for the power supply to downstream electronic circuits (not shown).
To be able to maintain internal normal d.c. voltage VCC for at least a short period of time in the event of failure of vehicle electric system supply voltage VBAT so that at least some of the downstream electronic circuits will be able to continue operating satisfactorily, the known circuit arrangement includes an energy storage mechanism designed as a capacitor 3, which is charged to a voltage distinctly higher than vehicle electric system supply voltage VBAT during regular operation to minimize the size of capacitor 3 for cost reasons and nevertheless store as much power as possible in it, and thus be able to bridge the gap for the longest possible period of time in the event of an emergency. To generate a much higher charging voltage for capacitor 3 from vehicle electric system supply voltage VBAT, a step-up regulator 5 is situated between non-reversible diode 1 and capacitor 3, the output voltage of this step-up regulator functioning in regular operation as the charging voltage for capacitor 3 and also as input direct voltage VZP for a step-down regulator 7, which generates from this input direct voltage VZP normal d.c. voltage VCC which is actually needed.
In an emergency, from reserve voltage VRES which is supplied by capacitor 3 and is high at first but then drops continuously, this step-down regulator 7 maintains normal d.c. voltage VCC for as long as possible.
Step-down regulator 7 is part of a closed loop including a comparator (not shown) which compares internal normal d.c. voltage VCC with a predetermined setpoint value and including a final controlling element (not shown) which supplies step-down regulator 7 with a control signal that varies as a function of the voltage difference detected by the comparator.
Two fundamentally different types of such step-down regulators are known.
First, a linear regulator may be used here, including a longitudinal transistor whose on-state voltage is varied so that it is equal to the required difference between input direct voltage VZP and internal normal d.c. voltage VCC which is to be established. This procedure takes place in regular operation and also during emergency operation in which input direct voltage VZP is then equal to progressively declining reserve voltage VRES delivered by capacitor 3. The simple design of such a linear regulator is advantageous, but it has the disadvantage that during emergency operation it causes a power loss for at least as long as reserve voltage VRES is significantly higher than internal normal d.c. voltage VCC that is to be established.
As an alternative, a switching regulator whose longitudinal transistor is alternately completely conducting or completely blocked with the help of triggering pulses may also be used here. Depending on the size of input direct voltage VZP, the pulse duty factor of the triggering pulses is varied, so that required internal normal d.c. voltage VCC is obtained at a downstream smoothing capacitor. This is advantageous in emergency operation in particular because the power losses remain low regardless of the size of input direct voltage VZP=VRES, but this comes at the expense of a much greater circuit complexity.
Step-up regulator 5 is required in the related art but it has a number of other disadvantages apart from the complex circuitry and the increased space required. For example, it requires an inductance which is difficult to implement as an integrated circuit and it creates an additional power loss which has a negative effect on the efficiency of the arrangement and increases the complexity for dissipation of the heat thus generated. It also has a negative effect on the EMC (electromagnetic compatibility) properties of the arrangement because it increases EMC emissions.